How wasp queens survive BC winters: overwintering biology explained

The annual colony death cycle

Every wasp colony you encounter in Metro Vancouver — yellowjacket, paper wasp, or bald-faced hornet — is a single season's work. The colony starts in spring when a mated queen emerges from her winter shelter, finds a nest site, and begins building alone. It peaks in August with hundreds to thousands of workers. It dies in September and October as temperatures drop and the colony's brood-rearing function ends. By November, the nest is completely abandoned — no living wasp remains inside it. The workers die. The queen who started the colony dies. New queens and males produced in late summer mate in autumn and then the males die; the mated queens are the only individuals that carry the genetic line through to next spring.

Where queens overwinter in Metro Vancouver

The overwintering queen requires three things from her shelter: insulation from freezing temperatures, protection from moisture, and concealment from predators. Metro Vancouver's climate is mild enough that true lethal freezing temperatures are uncommon below the snowline — a queen tucked under bark or inside a wall cavity rarely experiences temperatures below -5°C, which is below the lethal threshold for adult wasps but infrequently reached in coastal BC. The most common overwintering sites in our inspection experience are: the interface between bark and wood on large-diameter garden trees and stumps, the insulation layer in attic voids (fiberglass batting provides both insulation and desiccation protection), gaps in exterior wall assemblies at the stud-sheathing interface, under-eave cavities with southern exposure, and — commonly misunderstood — inside the dead woody stems of ornamental shrubs.

• Under tree bark and in rotting wood — the most natural overwintering site; queens enter through bark gaps and enter a cold-induced torpor in the woody tissue interface.
• Attic insulation — fiberglass batts provide excellent insulation and desiccation; queens enter through gable vents and overwinter in the insulation edge near the eave.
• Wall cavity interface — the air gap between exterior sheathing and interior drywall maintains a more stable temperature than the outside surface; queens enter through soffit gaps and weathering cracks.
• Ornamental shrub stems — dead or hollow stems of large ornamental shrubs (wisteria, climbing roses, mature rosemary) are commonly found with overwintering queens in Metro Vancouver inspections.
• Under-deck spaces — the soil-to-deck interface on older decks with loose board assemblies provides protected low-temperature cavities.
• Soil cavities — queens occasionally overwinter below the frost depth in the same genus of soil cavity yellowjackets use for summer nesting.

The overwintering physiology

Overwintering in insects relies on one of two strategies: freeze tolerance (the insect survives tissue freezing) or freeze avoidance (the insect supercools, preventing ice crystal formation). BC wasp queens primarily use freeze avoidance. Before entering winter dormancy, the queen reduces her body water content and accumulates cryoprotectant compounds (glycerol and other polyols) in her hemolymph. These compounds lower the supercooling point — the temperature at which ice crystal formation begins — to approximately -12°C to -17°C in well-conditioned queens. Metro Vancouver's coastal climate rarely drops below these temperatures, making the survival rate of overwintering queens here higher than in the BC interior or Alberta, which is one reason why Metro Vancouver has higher wasp pressure per hectare than most of Canada.

The dormancy period is not complete hibernation — on warm days (above 7-10°C) queens can arouse, move slightly within their shelter, and occasionally emerge to feed on carbohydrate sources (tree sap, early flowers) before returning to dormancy. This partial arousal is common in Metro Vancouver's mild winters and is why homeowners occasionally find individual wasps on warm January or February days — usually queens disturbed from their overwintering site who are briefly active before the main spring emergence.

Spring emergence and the critical founding period

When spring temperatures consistently reach 10-12°C for several consecutive days, the queen completes arousal, begins consuming carbohydrates intensively, and starts scouting for a nest site. She has roughly 4-6 weeks of this founding period where she is building the nest alone and raising the first cohort of workers — and this is the most dangerous period of her life. She has no workers to defend her. She is foraging for food, water, and nest material while simultaneously building and incubating eggs. Mortality is high: surveys of founding yellowjacket queens suggest 85-90% do not produce a successful colony. Predation by birds and spiders, failure to find adequate food, and adverse weather during this critical window all take a toll. The queens that survive this founding period and get their first cohort of workers past the 20-worker threshold have a high probability of going on to produce a full colony.

Why the prevention window is March

The founding period — those 4-6 weeks when a queen is evaluating and starting a nest alone — is the only point in the colony cycle when preventive treatment is effective. A residual pyrethroid applied to potential nesting surfaces in March creates a chemical environment that founding queens avoid. Once a colony has even 20 workers, the chemical deterrent effect is much weaker than the colony's investment in the existing nest — they will stay and defend rather than abandon. This is why early March applications matter and April applications are marginal. For Metro Vancouver homeowners who have had recurring problems at a specific location, booking a March residual treatment is the highest-value single intervention available.